I get motion sickness from riding my golf cart to the office if the roads have pot holes! I feel like Doctor Seuss when I say that I feel ill on a train on a boat on a plane in a bus in a helicopter - and yes, also when I went skydiving and was in free fall - you name it.
HOWEVER - I don't let that stop me from going places and doing fun activities. I used to always take dramamine and have now figured out exactly how much I need to cut the pills down in order to not get ill and not too drowsy. I have also recently begun using some homeopathic remedies that work - without the drowsiness. When I travel I warn the folks around me that I have taken travel sickness medication so that they need to excuse me if I am not wild and bubbly and not to be upset if I simply drift off to sleep.
The ride to and from the Manatee areas are fairly fast paced and don't tend to induce motion sickness, and when they slow down to look for or watch the manatee the boats are usually in calm waters.
When you go to Hol Chan tell the guide that you are prone to travel sickness and he can be sure that you are the first one into the water and the last one out (less wave action when you are in the water.)
Ginger ale is a cure all for motion sickness too - but the affects are short-lived.
Don't let your vacation pass you by because of motion sickness - try the remedies and enjoy every last minute.